While much have been published or patented on the subject of PMA based catalysts and their uses as oxidation catalysts and, regeneration of other catalysts is generally known, very little prior work directed toward the regeneration of PMA based catalysts has been reported. Two references of which we are aware involve Japanese authors. One of these, a Japanese patent application, No. 77/29,660, filed by Mitsubishi Rayon Co., Ltd., describes the regeneration of a PMA based catalyst by treatment with ammonium hydroxide, and hydrogen peroxide or ozone and optionally also with nitric acid or ammonium nitrate. The inventors reported that the initial oxidation of methacrolein with the fresh catalyst gave 88% methacrylic acid with 66% conversion of methacrolein; the spent catalyst gave only 71.5% methacrylic acid with 30.3% conversion of methacrolein and, that following regeneration with ammonium nitrate and ammonium hydroxide and 5 parts of 30% hydrogen peroxide for 30 minutes at 90.degree. C., the catalyst gave 88.1% methacrylic acid with 65.1% conversion of methacrolein. This process is undesirable inasmuch as it merely attempts to oxidize the deactivated catalyst and to do so outside of the reactor, necessitating shut-down.
The second of the references is another Japanese patent application, No. 77/66,629, filed by Japan Synthetic Rubber Co., Ltd., and describes the regeneration of spent catalysts containing oxides of molybdenum, phosphorous and vanadium and one or more of potassium, rubidium, cesium and thallium utilized in the oxidation of methacrolein to methacrylic acid. Regeneration was accomplished by treating with hydrochloric acid at 0.degree. to 200.degree. C. for more than 30 minutes followed by calcining in air or oxygen. The fresh catalyst initially gave 55.3% yield of methacrylic acid with 85% conversion of methacrolein; for the spent catalyst, yield and conversion dropped to 37.4% and 53.1%, respectively; however, following regeneration of the catalyst, yield and conversion were increased to 54% and 81.2%, respectively. Unfortunately, this process includes operation in the liquid phase and is therefore not as desirable as regeneration in the vapor phase which can take place within the reactor.
To be useful, we believe the regeneration of PMA based catalysts must occur in the vapor phase, and whether inside or outside of the reactor, without catalyst removal. It is also necessary to reactivate the catalyst in a manner that is compatible with a fluid-bed process. In such an instance, even though the life of the catalyst were on the order of one month, only a small slip stream sufficient to turn the charge over several times per month would be sufficient for regeneration. The regenerator could actually be quite small and fabricated from conventional corrosion resistant materials.